Medical magnetic resonance studies are typically carried out in strong magnetic fields. In an electromagnet employing normal conductors, a portion of the electrical power used to generate the magnetic field is consumed in heating of the resistive coil conductor. Thus, many kilowatts of power may be required to produce the magnetic field strength for a volume sufficient to develop a magnetic resonance image for medical diagnostics.
A coil of a conductive material, which is wound into a solenoid, generates the magnetic field for the magnet. The use of superconducting coils in MRI systems greatly reduces the amount of power consumed since a superconductor has a resistance which approaches zero and thus does not expend as much power to produce the same magnetic field. One of the controlling factors in superconductivity is the critical temperature of the conductive material of the coil. In a typical superconducting MRI magnet, a cryogenic fluid is used to cool a superconducting coil to a temperature at or below the critical temperature so that the coil exhibits superconducting properties.
One type of MRI magnet incorporates a frame formed from a ferromagnetic material, disclosed in U.S. Pat. Nos. 4,766,378 and 5,754,085, the disclosures of which are hereby incorporated by reference herein.
The MRI magnet disclosed in certain embodiments of U.S. Pat. No. 5,754,085 includes a first superconducting coil assembly supported on an upper support and a second superconducting coil assembly supported on a lower support. Superconducting coil assemblies typically include a container for the superconducting coil and the coil is disposed inside the container and immersed in a bath of liquid helium held in the container.
A substantial field is created by the primary magnet assembly. The net forces acting on the coils results in attraction of the coils to each other or attraction between the individual coils and the adjacent ferromagnetic components. The direction and magnitude of the force depends on the location of the coils with respect to the median plane of the magnet. The forces created by the field tends to spread the coil in a radial direction, tending to unravel the coil, or causes the coils to be attracted to the adjacent ferromagnetic structure. Thus, mounting and supporting the superconducting coils within an MRI magnet is a substantial problem. In addition, one of the issues which must be addressed is the tendency of the support structure for the superconducting coil to add heat to the coil by conduction from the outside environment.
The cryogenic fluid utilized to cool a superconducting coil is costly, which is a drawback as compared to a non-superconducting electromagnet. An MRI magnet including support structure for the superconducting coil which minimizes the amount of heat introduced into the superconducting coil, and which utilizes a minimum amount of cryogenic fluid is desirable for reasons of operating cost. Moreover, it is desirable to minimize the amount of cryogenic fluid in proximity to the coil for other reasons. A superconducting magnet coil can become normally-conducting in certain unusual circumstances known as a "quench". When this happens, energy stored in the flowing current is dissipated rapidly, and the surrounding cryogenic liquid is converted to a gas. While a properly designed superconducting magnet should not quench, safety precautions should be provided to take into account possible quenching and gas evolution. With less cryogenic fluid in the superconducting coil assembly, this problem is minimized.